1. Field of the Invention
This invention relates generally to a developer storage and delivery system, and more particularly concerns storing a phase change developer on a continuous web or on an endless belt and delivering the phase change developer to a liquid electrophotographic developing system.
2. Background of the Art
In electrophotography, a photoreceptor in the form of a plate, belt, or drum having an electrically insulating photoconductive element on an electrically conductive substrate is imaged by first uniformly electrostatically charging the surface of the photoconductive element, and then exposing the charged surface to a pattern of light. The light exposure selectively dissipates the charge in the illuminated areas, thereby forming a pattern of charged and uncharged areas (i.e., an electrostatic latent image). A liquid or dry developer is then deposited in either the charged or uncharged areas to create a toned image on the surface of the photoconductive element. The resulting visible image can be fixed to the photoreceptor surface or transferred to a surface of an intermediate transfer material or a suitable receiving medium such as sheets of material, including, for example, paper, polymer, transparency, metal, metal coated substrates, composites and the like. The imaging process can be repeated many times on the reuseable photoconductive element.
In some electrophotographic imaging systems, the latent images are formed and developed on top of one another in a common imaging region of the photoreceptor. The latent images can also be formed and developed in multiple passes of the photoreceptor around a continuous transport path (i.e., a multi-pass system). Alternatively, the latent images can be formed and developed in a single pass of the photoreceptor around the continuous transport path. A single-pass system enables the multi-color images to be assembled at extremely high speeds relative to the multi-pass system. At each color development station, color developers are applied to the photoreceptor belt, for example, by electrically biased rotating developer rolls.
Image developing methods can be classified into liquid type and dry type. The dry type method uses dry (e.g., powder) developers and the wet type method uses liquid developers.
Dry developers are generally prepared by mixing and dispersing colorant particles and a charge director into a thermoplastic binder resin. This mixing and dispersing is followed by milling or micropulverization. The resulted developer often comprises a powder having particle sizes that are generally in the range of about 4 to 10 microns. If the fine powder of a dry developer is scattered, it poses an environmental problem because of its small particle size. Therefore, most dry developers are stored in a cartridge which is easily handled and disposed of Furthermore, the stability of dry developer is usually much better than that of liquid developer.
Liquid developers are usually prepared by dispersing colorant particles, a charge director, and a binder in an insulating liquid (i.e., a carrier or a vehicle). Liquid developer based imaging systems incorporate many features similar to those of dry developer based system. However, liquid developer particles are significantly smaller than dry developer particles. Because of their small particle size, ranging from 3 microns to submicron size, liquid developers are capable of producing very high resolution images. However, liquid developers have some drawbacks.
The major drawbacks of liquid developers are (1) the emission of the liquid carrier from liquid developers to the environment during the drying and transfer process due to inefficient solvent recovery system; (2) the need and difficulty in disposing the waste liquids; (3) the inconvenience of using and handling of liquid developers; (4) and the aggregation and sedimentation instability of (the materials inside the developer are stable, both individually and in their association with other materials in the developer, e.g., non-reactive) liquid developers.
While known liquid developers and processes are suitable for their intended purposes, a need remains for liquid developers and processes that reduce or substantially eliminate the above-mentioned drawbacks. Additionally, there is a need for liquid developers and processes that enable the formation of high quality images on a wide variety of substrates.
There have been many attempts to solve some of the above-mentioned drawbacks of liquid developers and dry developers. For example, U.S. Pat. No. 5,075,735 to Tsuchiya et al. discloses a developer delivery system comprising stripes or bars of solid developer mounted across a belt. The stripes or bars of solid developer are caused to drop onto a heater by a cutter and then the solid developer is melted by the heater into liquid. The resulted liquid developer is then used to develop electrophotographic images.
U.S. Pat. No. 5,815,780 to Boerger et al. discloses an apparatus for storing and delivering toner. The toner is stored on a belt in discretely sealed toner bubbles filled with toner. An extractor unit then causes toner bubbles to rupture, allowing the toner to fall into a developer housing to replenish the toner supply.
U.S. Pat. No. 5,998,081 to Morrison et al. discloses a metallic web coated with a solid developer which is melted by an external conductive heating element. The melted developer is caused to form visible images by contacting with electrostatic latent images.
This invention provides an improved developer storage and delivery system which eliminates or reduces the above-mentioned drawbacks of liquid developers and processes while providing high quality images on a wide variety of substrates.
In a first aspect, the invention features a developer storage and delivery system for liquid electrophotography that includes:
a conductive substrate with a first surface and a second surface;
a plurality of discrete conductive heating elements mounted on said first surface; and
a phase change developer having a melting point of at least 22xc2x0 C., wherein said phase change developer is on the top surface of each of said conductive heating elements, except that a minor portion of the top surface of each of said conductive heating elements is free of said phase change developer. The term minor portion is used in its normal sense as less than 50% of the surface area directly over the conductive stripes is free of the phase change developer. It is preferred that this minor area be a small area, defined herein as less than 20% of the surface area over the conductive stripes or of the entire surface of the developer system, for example, 0.05 or 0.1% to 20%, 1 to 15%, 0.2 to 10%, 0.1 to 5% and 0.1 to 2% of the surface area over the conductive stripes or the developer system. The conductive stripes are for conducting electricity, preferably as part of a resistive heating element to heat the phase change developer. The developer is not necessarily conductive, and there must be at least a minor area and preferably a small area (as defined above) that is exposed to enable external electrical contact to connect the resistive heating element with an external power source.
In a second aspect, the invention features a developer storage and delivery system for liquid electrophotography that includes:
a conductive substrate with a first surface and a second surface;
a plurality of discrete conductive heating elements mounted on said first surface; and
a phase change developer having a melting point of at least 22xc2x0 C., wherein said phase change developer is a continuous layer on the top surface of each of said conductive heating elements and on said first surface free of said conductive heating elements, except that a small of the top surface of each of said conductive heating elements is free of said phase change developer for conducting electricity.
In a third aspect, the invention features a developer storage and delivery system for liquid electrophotography that includes:
an insulating substrate with a first surface and a second surface;
a plurality of discrete conductive heating elements mounted on said first surface; and
a phase change developer having a melting point of at least 22xc2x0 C., wherein said phase change developer is placed on the top surface of each of said discrete conductive heating elements, except that a small portion of both ends of each of said conductive heating elements is free of said phase change developer for conducting electricity.
In a fourth aspect, the invention features a developer storage and delivery system for liquid electrophotography that includes:
an insulating substrate with a first surface and a second surface;
a plurality of discrete conductive heating elements mounted on said first surface; and
a phase change developer having a melting point of at least 22xc2x0 C., wherein said phase change developer forms a layer that is preferably a continuous layer (This embodiment has a continuous developer layer on top of both the substrate and the heating elements, this structure shown primarily because it is easy to manufacture. This embodiment is shown in FIG. 2. Non-continuous coatings may comprise, for example only, porous, patterned, striped, etc. coatings. However, we prefer the developer to be continuous so that no area of the developer roll is uncovered) on the top surface of each of said conductive heating elements and on said first surface free of said conductive heating elements, except that a small portion of both ends of each of said conductive heating elements is free of said phase change developer for conducting electricity.
The term xe2x80x9cphase change developerxe2x80x9d has an accepted meaning within the imaging art, however, some additional comments are useful in view of phenemic differences amongst mechanisms in this field. As the term indicates, the developer system is present as one physical phase under storage conditions (e.g., usually a solid) and transitions into another phase during development (usually a liquid phase), usually under the influence of heat or other directed energy sources. There are basically two preferred mechanisms in which these phase changes appear: a) complete conversion of the phase change developer layer from a solid to a liquid and b) release of a liquid from a phase change developer layer with a solid carrier in the phase change developer layer remaining as a solid during and after development. The first system operates by the entire layer softening to a point where the entire layer flows, carrying the active developer component to the charge distributed areas and depositing the developer composition on the appropriate areas where the charges attract the developer. In this case, the developer may be originally or finally in a solid phase or liquid phase within the phase change developer layer, but with the softened (flowable or liquefied) layer carrying the developer or allowing the developer to move over the surface of the layer having image-effecting charge distribution over its surface. The second system, where a liquid developer forms on the surface of the phase change developer carrying layer, usually maintains a solid carrying layer with a liquid developer provided on the surface of the carrier layer. This system may function, for example, by the developer having a lower softening point or even being present as a liquid (e.g., liquid/solid dispersion, liquid/solid emulsion) in the solid carrier layer. Upon activation or stimulation (e,g, by energy, such as heat), the developer composition will exude or otherwise emit from the surface of the solid carrier. This can occur by a number of different phenomena, and the practice of the invention is not limited to any specifically described phenomenon. For example, a phase change developer layer may be constructed by blending a developer composition that is solid at 22xc2x0 C., which may be dispersed in a solid binder that is solid at 70xc2x0 C., and the phase change developer composition coated on the imaging surface. Upon heating of the phase change developer layer to a temperature between 25xc2x0 C. and 65xc2x0 C., for example, especially where the developer composition is present at from 1 to 60% by weight of the phase change developer layer, the developer will soften or liquefy, and the developer composition will flow to the surface of the developer layer. The developer may be present as droplets and spread by physical action or may flow in sufficient volume to wet the surface of the developer layer and form a continuous layer of liquid. Thus, in the practice of the present invention, the phase change developer layer may be heated above room temperature and below or above the melt, softening or flow temperature of the carrier solid in the phase change developer layer.
The concept of an xe2x80x98activation pointxe2x80x99 or xe2x80x98activation temperaturexe2x80x99 is particularly easily understood in the concept of the present invention. At room temperature, below the activation temperature, the phase change developer layer will not allow the developer to readily distribute over the differentially charged layer to form a pattern or latent image or image in response to the distribution of charges. When the activation temperature has been exceeded on the phase change developer layer, the developer becomes able to be distributed over the differentially charged layer to form a pattern or latent image or image in response to the distribution of charges. The activation point or activation temperature is therefore the temperature at which the phase change developer layer passes from a state in which the developer is electrophotographically inactive to a state where the developer is electrophotographically active, as the temperature increases.
The developer storage and delivery system of the present invention will be described primarily with respect to electrophotographic office printing; however, it is to be understood that these developers are not so limited in their utility and may also be employed in other imaging processes, other printing processes, or other developer transfer processes, such as high speed printing presses, photocopying apparatus, microfilm reproduction devices, facsimile printing, ink jet printers, instrument recording devices, and the like.